Insensitivity to the spatial repellent action of transfluthrin in Aedes aegypti: a heritable trait associated with decreased insecticide susceptibility.

Wagman JM, Achee NL, Grieco JP - PLoS Negl Trop Dis (2015)

Bottom Line:
Reported here are findings from a series of in vitro studies that investigated the plasticity and heritability of spatial repellent (SR) behaviors in Aedes aegypti exposed to airborne transfluthrin, including results that indicate a possible link between repellent insensitivity and insecticide resistance.Selective breeding of SR responders for nine generations did not change the proportion of mosquitoes repelled in any generation.Results also highlight the critical need to develop new repellent active ingredients with novel mechanisms of action.

Affiliation: Department of Preventive Medicine and Biometrics, Division of Tropical Public Health, Uniformed Services University of the Health Sciences, Bethesda, Maryland, United States of America.

ABSTRACT

Background: New vector control paradigms expanding the use of spatial repellents are promising, but there are many gaps in our knowledge about how repellents work and how their long-term use might affect vector populations over time. Reported here are findings from a series of in vitro studies that investigated the plasticity and heritability of spatial repellent (SR) behaviors in Aedes aegypti exposed to airborne transfluthrin, including results that indicate a possible link between repellent insensitivity and insecticide resistance.

Methodology/principal findings: A dual-choice chamber system was used to observe directional flight behaviors in Aedes aegypti mosquitoes exposed to passively emanating transfluthrin vapors (1.35 mg/m3). Individual SR responder and SR non-responder mosquitoes were identified, collected and maintained separately according to their observed phenotype. Subsequent testing included re-evaluation of behavioral responses in some mosquito cohorts as well as testing the progeny of selectively bred responder and non-responder mosquito strains through nine generations. At baseline (F0 generation), transfluthrin actively repelled mosquitoes in the assay system. F0 mosquitoes repelled upon initial exposure to transfluthrin vapors were no more likely to be repelled again by subsequent exposure 24 h later, but repelled mosquitoes allowed to rest for 48 h were subsequently repelled at a higher proportion than was observed at baseline. Selective breeding of SR responders for nine generations did not change the proportion of mosquitoes repelled in any generation. However, selective breeding of SR non-responders did produce, after four generations, a strain of mosquitoes that was insensitive to the SR activity of transfluthrin. Compared to the SR responder strain, the SR insensitive strain also demonstrated decreased susceptibility to transfluthrin toxicity in CDC bottle bioassays and a higher frequency of the V1016Ikdr mutation.

Conclusions/significance: SR responses to volatile transfluthrin are complex behaviors with multiple determinants in Ae. aegypti. Results indicate a role for neurotoxic irritation of mosquitoes by sub-lethal doses of airborne chemical as a mechanism by which transfluthrin can produce SR behaviors in mosquitoes. Accordingly, how prolonged exposure to sub-lethal doses of volatile pyrethroids might impact insecticide resistance in natural vector populations, and how already resistant populations might respond to a given repellent in the field, are important considerations that warrant further monitoring and study. Results also highlight the critical need to develop new repellent active ingredients with novel mechanisms of action.

pntd.0003726.g002: Plasticity of spatial repellency behaviors.Weighted spatial activity index (SAI) scores for cohorts of Aedes aegypti females exposed to 1.35mg/m3 transfluthrin. After observing baseline (Day 1) behaviors, test mosquitoes were re-assayed on a subsequent day following either 24 or 48 hours of resting. * indicates a day 2 SAI significantly different than the baseline day 1 SAI, P<0.05.

Mentions:
Two variations of the behavioral plasticity experiment were performed using F0 mosquitoes, with differing results (Table 1 and Fig 2). During the first experiment, mosquito cohorts (total n = 180 mosquitoes, average baseline SAI = 0.08 ±0.03 SEM) were re-assayed after a 24 hour rest period and results indicated a large degree of plasticity in behavioral responses to the repellent: mosquitoes repelled on day one (n = 29) were not more likely to be repelled again on day two (SAI = 0.03 ± 0.02) (Fig 2). Mosquitoes not repelled on day one (n = 129) were equally unlikely to be repelled on day two

pntd.0003726.g002: Plasticity of spatial repellency behaviors.Weighted spatial activity index (SAI) scores for cohorts of Aedes aegypti females exposed to 1.35mg/m3 transfluthrin. After observing baseline (Day 1) behaviors, test mosquitoes were re-assayed on a subsequent day following either 24 or 48 hours of resting. * indicates a day 2 SAI significantly different than the baseline day 1 SAI, P<0.05.

Mentions:
Two variations of the behavioral plasticity experiment were performed using F0 mosquitoes, with differing results (Table 1 and Fig 2). During the first experiment, mosquito cohorts (total n = 180 mosquitoes, average baseline SAI = 0.08 ±0.03 SEM) were re-assayed after a 24 hour rest period and results indicated a large degree of plasticity in behavioral responses to the repellent: mosquitoes repelled on day one (n = 29) were not more likely to be repelled again on day two (SAI = 0.03 ± 0.02) (Fig 2). Mosquitoes not repelled on day one (n = 129) were equally unlikely to be repelled on day two

Bottom Line:
Reported here are findings from a series of in vitro studies that investigated the plasticity and heritability of spatial repellent (SR) behaviors in Aedes aegypti exposed to airborne transfluthrin, including results that indicate a possible link between repellent insensitivity and insecticide resistance.Selective breeding of SR responders for nine generations did not change the proportion of mosquitoes repelled in any generation.Results also highlight the critical need to develop new repellent active ingredients with novel mechanisms of action.

Affiliation:
Department of Preventive Medicine and Biometrics, Division of Tropical Public Health, Uniformed Services University of the Health Sciences, Bethesda, Maryland, United States of America.

ABSTRACT

Background: New vector control paradigms expanding the use of spatial repellents are promising, but there are many gaps in our knowledge about how repellents work and how their long-term use might affect vector populations over time. Reported here are findings from a series of in vitro studies that investigated the plasticity and heritability of spatial repellent (SR) behaviors in Aedes aegypti exposed to airborne transfluthrin, including results that indicate a possible link between repellent insensitivity and insecticide resistance.

Methodology/principal findings: A dual-choice chamber system was used to observe directional flight behaviors in Aedes aegypti mosquitoes exposed to passively emanating transfluthrin vapors (1.35 mg/m3). Individual SR responder and SR non-responder mosquitoes were identified, collected and maintained separately according to their observed phenotype. Subsequent testing included re-evaluation of behavioral responses in some mosquito cohorts as well as testing the progeny of selectively bred responder and non-responder mosquito strains through nine generations. At baseline (F0 generation), transfluthrin actively repelled mosquitoes in the assay system. F0 mosquitoes repelled upon initial exposure to transfluthrin vapors were no more likely to be repelled again by subsequent exposure 24 h later, but repelled mosquitoes allowed to rest for 48 h were subsequently repelled at a higher proportion than was observed at baseline. Selective breeding of SR responders for nine generations did not change the proportion of mosquitoes repelled in any generation. However, selective breeding of SR non-responders did produce, after four generations, a strain of mosquitoes that was insensitive to the SR activity of transfluthrin. Compared to the SR responder strain, the SR insensitive strain also demonstrated decreased susceptibility to transfluthrin toxicity in CDC bottle bioassays and a higher frequency of the V1016Ikdr mutation.

Conclusions/significance: SR responses to volatile transfluthrin are complex behaviors with multiple determinants in Ae. aegypti. Results indicate a role for neurotoxic irritation of mosquitoes by sub-lethal doses of airborne chemical as a mechanism by which transfluthrin can produce SR behaviors in mosquitoes. Accordingly, how prolonged exposure to sub-lethal doses of volatile pyrethroids might impact insecticide resistance in natural vector populations, and how already resistant populations might respond to a given repellent in the field, are important considerations that warrant further monitoring and study. Results also highlight the critical need to develop new repellent active ingredients with novel mechanisms of action.